1. Field of the Invention
This invention concerns light source systems, and more specifically a back light module typically used in a liquid crystal display (LCD).
2. Description of the Prior Art
Users of portable electronic devices such as laptop and notebook computers, mobile phones and game devices expect the viewing screens of portable electronic devices to be large, clear and bright. Performance equal to that of cathode-ray tube (CRT) desktop monitors is desired. LCDs are one type of flat panel display (FPD) that can satisfy these expectations. However, because liquid crystals in an LCD are not self-luminescent, the LCD needs a back light module that offers sufficient luminance (brightness) for a planar surface.
Referring to FIG. 10, a conventional back light module 20 includes a light guide plate 22, a plurality of light emitting diodes (LEDs) 21 adjacent to one side of the light guide plate 22. Light beams emitted from the LEDs 21 enter the side of the light guide plate 22 and eventually emit uniformly from a top surface of the light guide plate 22.
Referring to FIG. 11 and FIG. 12, a light intensity of the LED 21 rapidly decreases according to an increase of emission angle. In fact, almost all of the light intensity is concentrated in a small range and in the vicinity of the 0 degree emission angle (i.e., direct emission). In other words, the LED 21 has a very small effective radiation angle. When the LED 21 irradiates the side of the light guide plate 22, some areas of the light guide plate 22 proximate/corresponding to midpoints between adjacent LEDs 21 receive almost no light. Thus darkened areas are formed on the top surface of the light guide plate 22. The back light module 20 does not provide uniform brightness for an entire area of the associated liquid crystal display panel.